But I just don't like her...

Hi lovelies,
I was in a bit of a predicament earlier so thought I'd share it with you lovely lot. As you all know I am married, and as a married woman I do try to get on with L's friends. Unfortunately most of them are just not very nice people and L does change when he is around them, but I still try to be nice whenever we see them. 
There is one friend of his that I like but he has a girlfriend who I just can't stand. I wanted to get along with her but when someone tells you that you are mental and don't want your fella to have any mates so shouldn't be getting married one week before the wedding, you are going to dislike them. Just to clarify I am not mental and this was said because SHE rang ME from Thailand (yes thailand) saying that her fella and L apparently hadn't gone back to her fellas house that night. She thought they were up to no good and I couldn't get hold of L. Previous to this L had gone out with his mate before and cheated on me a few times (yes I know I should have dumped him then but he promised he wouldn't do it again) so understandably I thought the worse (which then turned out to be true but I didn't find that out until we were married). I told her what I thought and said that's why I didn't really want L and mate going out alone (although I wouldn't stop him) so she came out with that.
She then tried to stop the mate from being best man at our wedding, eventually agreeing to let him come if they could leave straight after the speeches and if she could be at the top table. I agreed to this cos it was what L wanted but in every single wedding photo she is glaring at me. Everyone has commented on it. 
Anyway, L rang me today and asked me if I wanted to go to York with them in a couple of weeks time. I really didn't want to as I can't stand her but at the same time I want to be a good wife to L so I didn't know what to do for the best. If I agreed to go I would have to be nice to her which I don't want, I'm not horrible to her I just don't speak to her. But luckily for me I can't get the time of work so I was given a perfect excuse for not going without looking awful.

What would you all have done?

@prettyinpeach04
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